Apparatus for producing an electrical signal that is indicative of a temperature

Abstract

Apparatus for producing an electrical signal that is indicative of a temperature is disclosed, the apparatus comprising: a first thin-film transistor TFT comprising a first source, a first gate and a first drain, the first drain being configured to receive a reference current; and a second TFT comprising a second source, a second gate and a second drain, the first and second gates both being configured to receive the same gate voltage, wherein the first and second TFTs are configured such that a temperature dependence of the first TFT differs from a temperature dependence of the second TFT, such that an output current at the second TFT and the second drain is dependent on temperature. The temperature dependence of the output current can be controlled by selecting suitable design parameters for the first and second TFTs. A method of designing the apparatus to produce an output current with a target temperature dependence is also disclosed.

The invention claimed is: 
     
       1. Apparatus comprising:
 a first thin-film transistor TFT comprising a first source, a first gate and a first drain, the first drain being configured to receive a reference current; and 
 a second TFT comprising a second source, a second gate and a second drain, the first and second gates both being configured to receive the same gate voltage, 
 wherein the first and second TFTs are configured such that a temperature dependence of the first TFT differs from a temperature dependence of the second TFT, such that an output current at the second TFT and the second drain is dependent on temperature. 
 
     
     
       2. The apparatus of  claim 1 , wherein in isolation the first and second TFTs both have a positive temperature dependence or both have a negative temperature dependence, and
 wherein the first TFT is more strongly temperature-dependent than the second TFT with the result that the overall temperature dependence of the output current is negative, such that a magnitude of the output current decreases as a temperature of the first and second TFTs increases. 
 
     
     
       3. The apparatus of  claim 1 , wherein in isolation the first and second TFTs both have a negative temperature dependence, and
 wherein the second TFT is more strongly temperature-dependent than the first TFT with the result that the overall temperature dependence of the output current is negative, such that a magnitude of the output current decreases as a temperature of the first and second TFTs increases. 
 
     
     
       4. The apparatus of  claim 1 , wherein the first and second TFTs are source-gated transistors. 
     
     
       5. The apparatus of  claim 4 , wherein an area of overlap between the first source and the first gate, S1, is different to an area of overlap between the second source and the second gate, S2, such that the temperature dependence of the first TFT differs from the temperature dependence of the second TFT. 
     
     
       6. The apparatus of  claim 5 , wherein for each of the first and second TFTs a threshold source length exists beyond which increasing the source length has a negligible effect on the temperature dependence of the current flowing through the TFT, and
 wherein one of S1 and S2 is at or above the threshold source length, and the other one of S1 and S2 is below the threshold source length. 
 
     
     
       7. The apparatus of  claim 1 , wherein the first TFT and the second TFT have different compositions such that the temperature dependence of the first TFT differs from the temperature dependence of the second TFT. 
     
     
       8. The apparatus of  claim 1 , wherein the first gate and the second gate comprise respective parts of a single gate, such that the single gate acts as a common gate for the first and second TFTs. 
     
     
       9. The apparatus of  claim 1 , wherein the apparatus is configured to operate in a temperature range between an upper temperature threshold and a lower temperature threshold, and wherein the upper and lower temperature thresholds are temperatures within a linear portion of respective temperature-current curves of the first and second TFTs. 
     
     
       10. The apparatus of  claim 1 , comprising:
 a constant current source configured to supply a constant current to the first drain as the reference current. 
 
     
     
       11. The apparatus of  claim 1 , comprising:
 a load connected to the second TFT such that a current flowing through the load is dependent on the output current at the second drain. 
 
     
     
       12. The apparatus of  claim 1 , wherein the apparatus is configured to operate as a temperature sensor in which a magnitude of the output current at the second drain is indicative of a temperature of the first and second TFTs. 
     
     
       13. The apparatus of  claim 12 , comprising:
 a current measuring unit configured to measure the magnitude of the output current; and 
 a temperature determining unit configured to determine a temperature of the first and second TFTs in dependence on the current measured by the current measuring unit, 
 optionally wherein the temperature determining unit is configured to output the determined temperature. 
 
     
     
       14. The apparatus of  claim 1 , comprising:
 a temperature controller configured to raise or lower a temperature of the first and second TFTs, wherein the second TFT is configured to supply the output current to the temperature controller such that a magnitude of a heating or cooling effect produced by the temperature controller is dependent on a magnitude of the output current, 
 wherein the first and second TFTs and the temperature controller form a feedback loop such that the apparatus acts as a temperature regulating circuit configured to maintain an environment in which the first and second TFTs are located at a substantially constant temperature. 
 
     
     
       15. The apparatus of  claim 14 , wherein the temperature controller comprises an oscillator circuit configured such that an oscillating frequency of the oscillator circuit is dependent on the magnitude of the output current provided by the second TFT. 
     
     
       16. The apparatus of  claim 15 , wherein a Joule heating effect on the first and second TFTs due to current flowing through the oscillator circuit is dependent upon the magnitude of the output current provided by the second TFT, or
 wherein the apparatus comprises a temperature control circuit configured to receive an output of the oscillator circuit as a timing signal and produce a heating or cooling effect in dependence on the oscillating frequency of the timing signal, such that the heating or cooling effect produced by the temperature control circuit is dependent on a temperature of the first and second TFTs. 
 
     
     
       17. The apparatus of  claim 15 , wherein the oscillator circuit is a current-starved ring oscillator circuit and the second TFT is one of a plurality of second TFTs included in the current-starved ring oscillator circuit, each one of the plurality of second TFTs comprising a respective second gate configured to receive the same gate voltage as the first gate. 
     
     
       18. A wearable electronic device comprising the apparatus according to  claim 1 , wherein the apparatus is configured to regulate a temperature of at least part of the wearable electronic device. 
     
     
       19. A method of designing an apparatus according to  claim 1 , the method comprising:
 determining a target temperature dependence of the output current at the second TFT; 
 determining a temperature dependence of the first TFT and a temperature dependence of the second TFT required to provide an output current with the target temperature dependence; and 
 determining a property of the first TFT required to provide the determined temperature dependence of the first TFT, and determining a property of the second TFT required to provide the determined temperature dependence of the second TFT. 
 
     
     
       20. The method of  claim 19 , further comprising fabricating the designed apparatus.